1. Field of the Invention
The invention concerns a method to determine phase and/or amplitude between interfering, adjacent x-ray beams in a detector pixel in a Talbot interferometer for projective and tomographical x-ray phase contrast and/or x-ray dark field imaging.
2. Description of the Prior Art
It is known to determine phase and/or amplitude between interfering adjacent x-ray beams in a detector pixel in a Talbot interferometer for projective and tomographical x-ray phase contrast and/or x-ray dark field imaging by producing, after an irradiation of the examination subject with at least two coherent or quasi-coherent x-ray beams—an interference of the two coherent or quasi-coherent x-ray beams using an exposed phase grating, and the variation of multiple intensity measurements in temporal succession after an analysis grating is determined in relation to known relative shifts of a grating positioned in the beam path or of an x-ray source fashioned like a grating, relative to one or more of the gratings.
Similar methods to determine phase and amplitude between interfering, adjacent x-ray beams in a detector pixel are generally known. The disclosure document DE 10 2006 037 255 A1 is referenced as an example. In this document (in particular in FIGS. 1 and 2 with associated description) it is shown how the intensity curve of a detector pixel is to be measured depending on the displacement of an upstream x-ray grating in order to determine the relative phase shift of x-rays between two adjacent detector pixels. A grating upstream of a detector pixel is sequentially displaced perpendicular to the grating alignment, wherein at every displaced position the radiation intensity occurring at the detector pixel is measured. The intensity curve across the grating positions can be determined via at least three measurements at different grating positions, and the phase can therefore be calculated.
A significant problem in this type of measurement is that a sequential displacement of one of the gratings must occur. This means that the grating is displaced by a specific amount, and an integrating measurement of the radiation intensity at the detector pixel is subsequently implemented in a final time period, whereupon a displacement of the grating with subsequent measurement at an unmoved grating occurs again. This cycle is repeated until the maximum available shift of the grating is achieved. In the case of the examination of a patient, in order to minimize the patient dose the radiation source is alternatively switched on and off so that radiation is emitted toward the patient only during the integration time of the detector, thus when the grating is stationary and the actual measurement ensues. Such a method is very complicated and generates relatively long sampling measurement times, so it is difficult to integrate these measurement procedures into practical applications, in particular into fast CT scans with a rotating gantry.
Another possibility of direct measurement of phase and amplitude of the intensity curve of the radiation after an analysis grating is described in DE 10 2006 017 290 A1. Here the combination of analysis grating and detector pixel is replaced by the detector pixel itself having a number of detection strips fashioned in grating line directions so that the phase and amplitude of the corresponding x-ray can be directly measured in a measurement session. However, such an arrangement is very complicated and, at the present time, is not suitable for designing the detector in quantity, due to cost reasons.
European Patent Application EP 1 803 398 A1 discloses that instead of an absorption grating arranged at the source, an x-ray source is executed in a band shape that achieves the same effect as a source grating downstream of the focus. This variant of an x-ray source is also usable in connection with the present method, with the displacement of the bands imitating x-rays on the anode being equated with the displacement of the source grating.
An additional method, in which the knowledge of the intensity curve of x-ray radiation after an analysis grating is necessary, is dark field imaging with hard x-rays. Reference is made in this regard to the publication by F. Pfeiffer et al., “Hard X-ray dark field imaging using a grating interferometer”, Nature Materials Vol. 7, Pages 15 through 137, 1 Feb. 2008. This document describes how an x-ray dark field imaging (similar to dark field imaging from optical microscopy) is possible, wherein the information of the direct x-ray radiation is masked out using a grating interferometer and only the information of the scatter radiation is used for imaging. It is necessary to know the intensity curve depending on the displacement of an analysis grating in a detector pixel in order to determine the amplitude of the intensity curve on the basis of the measured intensity curve. Not only the phase information but also the amplitude information is used for imaging. In principle, a corresponding measurement of the intensity curve of the detector pixel is necessary depending on the displacement of an upstream grating. A sequential displacement of the grating and measurement at a stationary grating have also been previously implemented in this context. The difficulties that result due to such a sequential measurement correspond to the difficulties that occur in phase contrast measurement.